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Abstract: Tropical forests exhibit significant heterogeneity in plant functional and chemical traits that may contribute to spatial patterns of key soil biogeochemical processes, such as carbon storage and greenhouse gas emissions. Although tropical forests are the largest ecosystem source of nitrous oxide (N2O), drivers of spatial patterns within forests are poorly resolved. Here, we show that local variation in canopy foliar N, mapped by remote‐sensing image spectroscopy, correlates with patterns of soil N2O emission from a lowland tropical rainforest. We identified ten 0.25 ha plots (assemblages of 40–70 individual trees) in which average remotely‐sensed canopy N fell above or below the regional mean. The plots were located on a single minimally‐dissected terrace (<1 km2) where soil type, vegetation structure and climatic conditions were relatively constant. We measured N2O fluxes monthly for 1 yr and found that high canopy N species assemblages had on average three‐fold higher total mean N2O fluxes than nearby lower canopy N areas. These differences are consistent with strong differences in litter stoichiometry, nitrification rates and soil nitrate concentrations. Canopy N status was also associated with microbial community characteristics: lower canopy N plots had two‐fold greater soil fungal to bacterial ratios and a significantly lower abundance of ammonia‐oxidizing archaea, although genes associated with denitrification (nirS, nirK, nosZ) showed no relationship with N2O flux. Overall, landscape emissions from this ecosystem are at the lowest end of the spectrum reported for tropical forests, consist with multiple metrics indicating that these highly productive forests retain N tightly and have low plant‐available losses. These data point to connections between canopy and soil processes that have largely been overlooked as a driver of denitrification. Defining relationships between remotely‐sensed plant traits and soil processes offers the chance to map these processes at large scales, potentially increasing our ability to predict N2O emissions in heterogeneous landscapes. [ABSTRACT FROM AUTHOR]

Plant functional traits affect soil nutrient cycling in many ecosystems, but high levels of diversity make it challenging to identify their influence in tropical rainforests. Here, we used a litter manipulation experiment to demonstrate that differences in litter chemistry alone are sufficient to drive changes in soil nitrogen (N) cycling within a two-year period in a lowland tropical rainforest. Previously, we used airborne imaging spectroscopy to identify ten 0.25 ha plots on the Osa Peninsula of southwestern Costa Rica, each containing 10–20 emergent trees, with relatively high or low mean canopy N content (± 0.4 standard deviations of the landscape mean; n = 5 each). Plots with high canopy N content had higher soil inorganic N concentrations, faster net N cycling rates, and greater nitrous oxide (N 2 O) emissions. Because abiotic conditions were similar among the plot types, we hypothesized that litter inputs drove the formation of these high and low soil N patches. We tested this hypothesis with a reciprocal litter transplant experiment. After two years of monthly treatments, litter inputs from the high canopy N plots increased soil N availability in the low canopy N plots by 30% but had no measurable effects on net N cycling rates or N 2 O emissions. Surprisingly, litter inputs from the low canopy N plots increased soil N availability and net nitrification in the high canopy N plots. While somewhat counterintuitive from a canopy N perspective, these results may reflect a more nuanced effect of litter chemistry. Although low canopy N plot litter had lower N content than high canopy N plot litter, it leached relatively more and higher quality soluble carbon (C), which we speculate may have had a positive priming effect, releasing N from more recalcitrant compounds. Additionally, low canopy N plot litter supported higher rates of free-living N fixation, which could have directly increased soil N availability. Overall, our results show that tropical tree assemblages with differing foliar chemistry can directly influence soil properties through litter inputs, and that this effect may be mediated not only by litter N, but also relative C solubility and chemistry. • We conducted a two-year litter transplant experiment in a diverse lowland Costa Rican forest. • Litter from tree assemblages with high foliar N increased N availability in Nsoils. • Rainforest tree assemblages may generate N hotspots via differences in litter chemistry. [ABSTRACT FROM AUTHOR]

Though tropical forest ecosystems are among the largest natural sources of the potent greenhouse gas nitrous oxide (N 2 O), the spatial distribution of emissions across landscapes is often poorly resolved. Leaf cutter ants (LCA; Atta and Acromyrmex, Myrmicinae) are dominant herbivores throughout Central and South America, and influence multiple aspects of forest structure and function. In particular, their foraging creates spatial heterogeneity by concentrating large quantities of organic matter (including nitrogen, N) from the surrounding canopy into their colonies, and ultimately into colony refuse dumps. Here, we demonstrate that refuse piles created by LCA species Atta colombica in tropical rainforests of Costa Rica provide ideal conditions for extremely high rates of N 2 O production (high microbial biomass, potential denitrification enzyme activity, N content and anoxia) and may represent an unappreciated source of heterogeneity in tropical forest N 2 O emissions. Average instantaneous refuse pile N 2 O fluxes surpassed background emissions by more than three orders of magnitude (in some cases exceeding 80 000 µg N 2 O-N m -2 h -1 ) and generating fluxes comparable to or greater than those produced by engineered systems such as wastewater treatment tanks. Refuse-concentrating Atta species are ubiquitous in tropical forests, pastures and production ecosystems, and increase density strongly in response to disturbance. As such, LCA colonies may represent an unrecognized greenhouse gas point source throughout the Neotropics.